It is known to use near or supercritical carbon dioxide as an extractant (i.e. liquified vapor near or above its critical point used to extract impurities from an aqueous stream), to extract certain liquid organic solutes from their solvents. U.S. Pat. Nos. 4,349,415 and 4,375,387 disclose respectively, a process and apparatus for separating organic liquid solutes from their solvent mixtures, using liquid carbon dioxide as an extractant. The process and apparatus include a liquid-liquid extraction column and a distillation column having a single reboiler which receives heat provided through indirect heat exchange with recompressed vapor drawn from the overhead of the distillation column. The process as particularly described yields an aqueous raffinate (i.e. water or aqueous stream after purification), via countercurrent contact of upwardly flowing liquified carbon dioxide with a water/ethyl alcohol mixture.
A stated objective of the referenced patents is the reduction of the required energy input per unit of organic liquid extracted. These patents propose reduced energy consumption in recovering and recycling extractant, via a process and apparatus that uses solvent extractant vapor recompression and the enthalpy of overhead vapors from the distillation column as the source of reboiler heat.
However, while acknowledging the desirability of recovering and recycling the extractant used in the process of extracting organics, the referenced patents assume and essentially require minimal boiling point elevation of the solvent/organic(s) matrix involved in the distillation process. In exclusively using carbon dioxide as the extractant and ethyl alcohol as the liquid organic solute (i.e. organic stream that remains after extraction and removal of the extractant), the referenced patents rely heavily on favorably low boiling point elevation resultant from the presence of the solute. The prior art effectively dismisses as uneconomical, the processing of organic chemicals whose colligative properties with carbon dioxide result in a significant boiling point elevation of the solvent/solute mixture (referred to hereinafter "subject mixtures").
The referenced patents, and the prior art generally, do not contemplate or substantiate the efficiency of carbon dioxide extraction in processing organic chemicals that result in substantial boiling point elevations of solvent/solute mixtures. Although it is generally appreciated that separation and reuse of carbon dioxide is critical in a plant-scale liquid carbon dioxide extraction facility, prior art technology cannot process the subject mixtures, such as acetone/water, without the excessive loss of carbon dioxide with the extracted organics.
Thus, known carbon dioxide extraction processes are inadequate for processing a variety of organic compounds that are miscible with carbon dioxide and interfere with efficient distillation and recovery of the heat of vaporization and recompression. Prior art technology cannot adequately, effectively and/or economically extract important compounds, such as acetone or the like, in a liquid-liquid carbon dioxide extraction process where solvent reuse is desirable and where the subject mixtures appreciably elevate the boiling point of the solution.